Irrigating Using ETc

Adapted from the article “Orchard Irrigation 2013” by Richard P. Buchner & Allan E. Fulton, UCCE Farm Advisors, Tehama County in the April 2013 Sacramento Valley Prune News

Increasingly, orchard producers are using various methods of monitoring and science-based information to more precisely decide when to begin irrigating and how much water to apply in specific orchard settings. This is especially true for those with drip and microsprinkler irrigation systems which enable greater control of the water application rate.

Some growers track real-time, weekly estimates of orchard ET and in-season rainfall. This information can be related to the soil water holding capacity of specific orchard soils and to the specific water application rates of their irrigation systems to help estimate how much soil moisture storage has been depleted before irrigation begins and then determine how long to run their irrigation system to replenish a portion of the depleted soil moisture.

One strategy is to start irrigating when trees have used enough soil water to make room in the “soil water bank” to hold irrigation water. The challenge is to avoid water logging on the wet side and tree stress on the dry side. A fairly accurate estimate of soil moisture depletion can be made by adding up daily water use. Start summing daily water use when you know the soil profile is full.

Another way to approach the answer is to consider how much water can be applied per set time and start when at least that much water has been depleted. A convenient source of weekly, real-time estimates of crop ET for orchards can be found on-line. This information is also published weekly in several local newspapers throughout the northern Sacramento Valley.

Weekly estimates of crop ET and soil moisture depletion are based on real-time, regional weather conditions and other reasonable assumptions about orchard health and development. Actual soil moisture depletion in specific orchards is likely to be different. Direct measurement of crop water stress coupled with estimates of crop ET or soil water depletion can be used to monitor orchard water status and how trees integrate complex orchard environments as well as estimate the soil moisture depletion and the need for irrigation.

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